The present invention relates to a pneumatic tyre intended for use on a rim with an emergency-driving device, and/or in a vehicle drive where the pneumatic tire supports and drives a track.
For instance U.S. Pat. Nos. 3,028,900, 4,183,388 and 4,258,767 disclose emergency-driving wheels which are based on the provision of an inner supporting device or inner wheel serving to prevent a deflated tire from collapsing entirely. Such a supporting device reduces deformation of the tire sides, and thus considerably prolonges the distance that the tire can run flat. However, emergency driving with an empty tyre mounted on a rim with an emergency-driving device suffers from the disadvantage that the supporting device or inner wheel is of smaller circumference than the tire, resulting in a relative movement between the inside of the tire and the emergency-driving device. Such relative movement or slipping wears the inside of the tire which thus is rapidly worn down from within, with the risk that the tire may split up in the radial plane or the tread of the tire come loose so that continued driving usually becomes impossible.
In the above US patent specifications, it is suggested that a rubber layer be arranged on the outer tread of the emergency-driving device or an inner emergency-driving device made of nothing but rubber or plastic be provided. An alternative is to coat the inside of the tire with a low-friction layer of ethylene polyterephthalate or a suitable lubricant (U.S. Pat. No. 4,007,769 and SE-B-379,313) which is either applied to the inside of the tire when the wheel is mounted or distributed thereon when the tire becomes flat. In some cases, this distribution may be carried out by the lubricant being discharged from a container arranged in the wheel and designed to break at the occasion of emergency driving.
The use of emergency-driving devices of the type described in U.S Pat. No. 3,028,900 has shown that a large amount of heat is generated by the wear or slipping between the rubber layer arranged on the emergency-driving device and the inside of the pneumatic tire. With the rapid build-up of heat, there is a risk that the tire rapidly catches fire and is destroyed. This risk is probably less pronounced when the coating on the emergency-driving device or the inside of the tire consists of a layer of polytetrafluoroethylene, but the risk still remains.
In tests, the use of lubricants in the tire has given excellent results in emergency driving on consolidated roads. Tires equipped with emergency-driving devices are, however, mostly intended for cross-country vehicles, such as military vehicles. Thus, in emergency driving with punctured tires in e.g. deserts or other extremely dry and dusty areas, the lubricant dries very quickly, probably since it takes up the fine-grained dust in the surroundings. As a result, the lubricating effect is rapidly lost, and the tire is destroyed as if there had been no lubricant at all. The provision of ethylene polyterephthalate layers on the emergency-driving device or on the inside of the tire is impaired by the same drawback, since ethylene polyterephthalate has a poor wear resistance and thus cannot withstand the wear caused by dust and other solid particles penetrating into the punctured tire.
A further difficulty encountered in vehicle drives of the type comprising pneumatic tires and a track running over the tires and driven thereby in order to propel the vehicle, is the wear of the tire portions which come into contact with tire-supporting elements serving to position the tires in relation to the track. This is especially pronounced under hot and/or dry conditions.
The present invention aims at providing a pneumatic tire which has a higher resistance to inner or outer wear and, consequently, has a longer service life. The invention further aims at providing a wheel structure comprising a rim on which a pneumatic tire is mounted, and an inner wheel for emergency driving which is mounted on the rim in the pneumatic tire, said wheel structure being designed so as to acquire better emergency-driving properties, especially in deserts and other dry and dusty areas, than prior art wheel structures of this type.
In addition, the invention aims at providing a vehicle drive comprising a track and pneumatic tyres which cooperate therewith, said vehicle drive having a longer service life than prior art vehicle drives of this type.